As shown in, for example, FIGS. 1A, 1B and 1C, a metal pipe 2 housing an optical fiber 3 is stranded together with conductive wires 1 so as to provide an optical grounding wire.
Specifically, FIG. 1A shows an optical grounding wire prepared by concentrically stranding a plurality of conductive wires 1 around the metal pipe 2 housing the optical fiber 3. FIG. 1B shows an optical grounding wire prepared by forming a first layer consisting of a plurality of conductive wires 1 and a plurality of metal pipes 2 each housing the optical fiber 3 to surround an inner core 4 in the form of a metal wire consisting of, for example, steel, aluminum, etc. and concentrically stranding a plurality of additional conductive wires 1 to surround the first layer. Further, FIG. 1C shows an optical grounding wire prepared by forming a first layer consisting of a plurality of conductive wires 1 and a metal pipe 2 housing the optical fiber 3 to surround the conductive wire 1 acting as a core and concentrically stranding a plurality of additional conductive wires 1 to surround the first layer.
The metal pipe housing the optical fiber is prepared by a simultaneous insertion method in which a metal tape is formed into a tubular form and, at the same time, the optical fiber 3 is supplied so as to be loaded in the tubular body and, then, the seam of the tubular body is bonded. Alternatively, the metal pipe housing the optical fiber is prepared by an after-insertion method in which the optical fiber is inserted from the end portion into the metal pipe prepared in advance. In general, the metal pipe is prepared by using a stainless steel, copper, a copper alloy, etc. in view of the required mechanical strength, workability and corrosion resistance of the metal pipe.
In the conventional technique, a corrosion problem is brought about by a difference in ionization tendency between the metal constituting the metal pipe 2 and the metal constituting the conductive wire 1, e.g., aluminum or aluminum alloy.
A measure for overcoming the difficulty is disclosed in, for example, Japanese Patent Publication (Kokoku) No. 63-10805 or Japanese Patent Disclosure (Kokai) No. 8-69716. These prior arts teach that the outer surface of the metal pipe housing the optical fiber should be covered with a metal layer. To be more specific, JP '805 teaches that a metal layer consisting of, for example, aluminum is formed to cover the outer surface of the metal pipe housing the optical fiber by means of vacuum plating such as vacuum vapor deposition, sputtering, or ion plating.
On the other hand, JP '716 teaches a sintering method in which a metal powder is sintered on the outer surface of a metal pipe housing an optical fiber, or a chemical or electrochemical metal plating method utilizing electrolysis for forming an aluminum or aluminum alloy layer to cover the outer surface of the metal pipe.
Where the metal pipe housing an optical fiber is stranded together with conductive wires to provide an optical grounding wire, wear is brought about between the metal pipe housing the optical fiber and the conductive wires around the metal pipe because of the elongation-shrinkage of the pipe and wires caused by vibration or temperature change. Particularly, where a metal layer is formed to surround the outer surface of the metal pipe housing the optical fiber, required is a high wear resistance exceeding the level in the ordinary steel plate plating, making it necessary to determine the thickness of the metal layer in view of the wear. According to the research conducted by the present inventors, it has been found that required is a metal layer having a thickness exceeding 30 .mu.m.
In JP '805 and JP '716, however, no consideration is given to the wear of the metal layer and to the adhesion properties and productivity of the metal layer. Further, if an aluminum layer having a thickness exceeding 30 .mu.m is simply formed without taking any measure in the film forming method disclosed in these prior arts and in the other known film forming methods, the aluminum layer is hardly adhered to the base body of the metal pipe, resulting in failure to achieve a sufficient productivity such as a production efficiency and yield.
Also, where a metal layer is formed to cover the outer surface of the metal pipe housing the optical fiber, a surface defective portion is unavoidably formed on the metal layer in forming the metal layer. What should be noted is that a corrosion derived from a difference in ionization tendency between the base body of the metal pipe housing the optical fiber and the plated metal is generated in the surface defective portion. In order to overcome the corrosion problem, it is effective to increase the thickness of the metal layer. However, the present inventors have found that, for overcoming the corrosion problem by increasing the thickness of the metal layer, it is necessary to form the metal layer in a thickness exceeding 30 .mu.m, as already pointed out. Any of JP '805 and JP '716 does not refer to the defect of the metal layer. Also, if the metal layer is formed in a thickness exceeding 30 .mu.m, the productivity is lowered and the adhesion of the metal layer to the base body of the metal pipe is markedly lowered, as already pointed out. If the metal layer is poor in adhesion properties, the metal layer readily peels off so as to expose the base body of the metal pipe to the outside. It follows that, even if an aluminum or aluminum alloy layer of an optional thickness is formed to cover the surface of the metal pipe, the base body of the metal pipe is exposed to the outside, resulting in failure to obtain a sufficient corrosion resistance.
The present invention, which has been achieved in view of the problems described above, is intended to provide an optical fiber-housing metal pipe having the surface covered with an aluminum layer and further covered with an oxide layer exhibiting a wear resistance and a corrosion resistance, to provide an optical fiber-housing metal pipe having the surface covered with an aluminum alloy layer or an aluminum composite layer exhibiting a corrosion resistance and a wear resistance, and to provide an optical fiber-housing metal pipe having the surface covered with an aluminum or aluminum alloy layer exhibiting an corrosion resistance and having the surface of the aluminum or aluminum alloy layer smoothed so as to decrease the surface defect and to further improve the corrosion resistance, and a manufacturing method thereof.